01 July, 2020

Light reactions: make ATP + NADPH for Calvin cycle


Calvin cycle

Calvin cycle is a 3-step process


  • 1. Fixation: CO2 reacts with 5C molecule (RuBP)
    • catalyzed by Rubisco enzyme
    • new 6C split into 2 acids (3C each)


  • 2. ATP and NADPH turn 3C acids into sugars
    • reduction of acids


  • 3. Some 3C sugars recycled to make RuBP (5C)
    • one 3C sugar leaves to make glucose
    • recycling needs ATP

Making sugars requires several cycles

Rubisco - the enzyme that dominates carbon fixation






  • Most abundant enzyme in the world


  • Most CO2 converted into biomass is fixed by Rubisco


  • Large molecule : 16 polypeptides : 8 active sites

Rubisco: the clunky and slow carbon fixer




  • Only 3-10 reactions per second
    • limits photosynthesis


  • 20% error rate
    • gets worse at high temperatures


  • Also reacts with O2
    • O2 reaction called Photorespiration
    • uses ATP and NADPH to make CO2

C3 photosynthesis how did we get here…




  • Evolution doesn’t always create the best solution


  • Photorespiration is completely wasteful
    • 2x energy to produce the same amount of sugar than if Rubisco only reacted with CO2


  • Rubisco evolved high affinity for CO2 (80x > O2)
    • compare to the composition of the atmosphere…
    • compare to global warming…

Rubisco substrate choice

Photorespiration inpacts food security

Rubisco and plant evolution: How did we get here?

Plant evolution tied to atmospheric shifts in CO2:O2

Why has Rubisco not been replaced?


  • Evolution of enzymes difficult
    • trade-offs between activity and stability


  • Removing oxygen reaction breaks Rubisco
    • selection against modifying mutations


  • RubisCO → RubisC???
    • requires a decrease in fitness
    • plants are stuck with Rubisco


  • Natural selection increased affinity for CO2
    • makes it slow

Evolution tinkers with other parts of photosynthesis



  • Improved and recombined existing parts and pieces
    • work around the photorespiration issue


  • In response to environmental change
    • lower [CO2] & higher [O2]
    • high temperatures
    • water limitations


  • Evolution of new photosynthesis pathways
    • Allowed plants to exploit new habitats

Plants and humanity: Why does photorespiration matter?




  • Consumes 32% of ATP and 28% of NADH in C3 leaves
    • under current conditions


  • Crop productivity improvement not sufficient to meet 2x food production by 2050


  • Photorespiration decreases US soybean and wheat yields by 36% and 20%
    • loss of 148 trillion potential calories
    • would feed 203 million people for a year